Lubrication considerations for grounded aircraft

The aviation industry is facing a challenging time as travel restrictions persist and airplanes are grounded in an effort to stop the spread of COVID-19. According to Aviation Week Fleet Data Services, approximately 9,000 aircraft are now in short-term parked status and more than 3,000 aircraft have been put in long-term storage worldwide.

As airlines grapple with an unprecedented number of grounded aircraft — and for an indeterminable amount of time — preventative maintenance is top of mind. Airlines likely find themselves dedicating an abundance of resources to keep parked aircraft in near ready-to-fly condition, and there are several factors airlines should remain mindful of to help ensure the right measures are taken to safely return planes to the skies when the time comes. Lubrication is one often overlooked but critical aspect.

This article explores lubricant considerations for grounded aircraft and is intended to be an educational resource. It is essential that airlines follow the instructions included in aircraft OEM maintenance manuals.

Lubrication best practice — exercise the engine

When an aircraft is parked for an extended period, there is increased risk for deterioration of its components and structure if preservation procedures are not followed. These procedures include everything from covering windows to preserve the inside of the aircraft, to setting up ventilation and dehumidifiers and covering gaps in the airframe to prevent birds from nesting in the engines.

Not to be overlooked, it is also important to consider “exercising” aircraft engines to bring the oil to operating temperature if possible. This is done to evaporate water and renew the film of protective additives on the surfaces of engine components, and is done weekly, or sometimes every two weeks.

Ester-based aviation turbine lubricants absorb water from the atmosphere at every opportunity. Water contamination will cause hydrolysis of the esters to occur, which forms acids likely to damage engine components. The longer the oil is exposed to the atmosphere, the more water it absorbs, increasing the risk of forming acids.

The rate of water contamination is also dependent on temperature and ambient humidity. Occasionally running engines while grounded can help evaporate the water, however, this alone may not be enough. And in some instances, exercising engines to bring oil to operating temperature might not be possible at all due to aircraft location, proximity to other aircraft, etc. It is important to periodically test the oil for water (ppm) and monitor total acid number (TAN) and compare these to the levels set by the engine OEM.

If the lubricant is contaminated with water for an extended period, eventually the TAN value will exceed OEM recommended limits, indicating it is time to change the lubricant. Some engine OEMs have fluid condition limits that stipulate a water level less than 1000 ppm and a TAN of less than 2.0. These are good general guidelines, but airlines need to consult their engine manuals for specific guidance. It’s also worth noting that there is no “too low” figure. For instance, Mobil Jet™ Oil II gas turbine lubricant right out of the can will typically have around 200-300 ppm.

Grease storage and distribution

Much like exercising the engines, maintenance technicians may exercise the wheels in order to avoid flat spots by moving the airplane. Rolling tires also renews the grease coating on the associated wheel bearing components, which helps protect the bearings. This is important because wheel bearings are only re-greased when completely removed from the aircraft. Moving the airplane also flexes the landing gear which renews oil and grease films on landing gear struts and linkages.

Additionally, knowing that the usage of greases decreases during prolonged aircraft downtime, it’s important for airlines to properly store their greases and be mindful of shelf life and contamination warning signs. Here are some tips on how to effectively accomplish this:

• Grease containers should be stored indoors in dry, cool and clean environments with temperatures ranges from 0°C to 40°C (32°F to 104°F). If a stored grease is briefly exposed to severe temperatures or environmental conditions, technicians should consult their lubricant suppliers with concerns.
• If a grease container is opened, the grease needs to be used as soon as possible to avoid potential contamination or degradation — which is an added concern when aircraft are grounded.

Improper storage may lead to hydrolysis, a risk factor for grease just as it is for turbine oils. Some signs grease has been compromised include unusual darker color, abnormal consistency and a strong, unpleasant odor. Oil separation may also be observed, however, greases with normal bleed can easily be mixed back together. If there’s excessive oil bleed, the grease will harden and be unsuitable for use.

While proper storage can help prevent these compromises, maintenance technicians also need to be mindful of shelf life and whether a container is open already. Shelf life specifications can differ between aviation greases. The average industry shelf life of aviation greases is about three years; however, ExxonMobil’s Mobilgrease™ 33 and Mobilgrease™ 28 aviation greases offer extended shelf life for up to 10 years.

While aviation grease shelf life is listed as the “use by” date on the container, the listed shelf life recommendation for a grease is no longer applicable once the container is opened. Once opened, greases with ester base oils are vulnerable to hydrolysis from absorbed water contamination and should be used as quickly as possible. ExxonMobil’s Mobilgrease 33 and Mobilgrease 28 aviation greases do not have these oils and using these can ease concerns around grease degradation. Also keeping a lid on the opened grease container helps prevent the ingress of particulate contamination.

Additives for deep preservation

While airlines work to keep grounded aircraft parked only temporarily, some aircraft are being put in long-term storage, or deep preservation. Long-term preservation is normally done when there is no timeline to return an engine to service. This may mean the engine will be out of service for six months or longer, and this is often a permanent situation.

When preserving or “pickling” an engine, preservation additives need to be added to the lubricants to prevent corrosion. Adding a preservative chemical should be done with the water level and TAN in compliance with OEM guidelines.

Once the long-term preservative additive is mixed in the lubricant, the engine operation is typically limited by the OEM as the preservative may interfere with other lubricant additives and create less load-carrying, or more deposits forming. Generally, once the preservative gets mixed and circulated, the engine sits without being operated until it is about to be put back into service, whenever that may be. Airlines know best when it is appropriate for them to use preservation fluid in engines as the workload to get a preserved engine back to airworthy status is a significant task.

The road to recovery

In today’s environment, there are a lot of unknowns, and this is especially true for the aviation industry. It could be that at some point a higher percentage of aircraft will be migrated from short-term storage to long-term preservation, and some will return to service. While much is unknown, airlines can maximize the effectiveness of their short-term storage effort by remaining mindful of lubricant-related risks, following OEM provided preservation requirements and taking the appropriate mitigation steps. These tips can also help ensure the airworthiness of an aircraft returned to service after extended downtime.

For anyone needing lubrication support during this time, ExxonMobil’s technical engineers are standing by to answer your questions and provide support. If you need assistance, please contact us.

“Lubrication Considerations for Grounded Aircraft” first appeared in the July 2020 issue of Aviation Maintenance Magazine. Reprinted with permission.